Need help. Not sure what will happen.

I have 2 iPhones. One is the older 3GS and my new 4S. Both are jailbroken but my older version is using an older JB (Not 5.x).
My question is this. I want to get rid of my older model and give it to a friend for her son. Now I'd like to restore it as new to clean it up for him. BOTH phones are linked to my 1 iphone "account" for a lack of a better word in iTunes. I want to know if I take the 3GS phone connect it to iTunes and restore as new will that mess up my 4S when I plug that one back in and backup in the future. Both phones have their own latest back ups. Or when I plug in the new 4S phone in the near future to backup/synch it'll just download all my data back into the iTunes account from the data that's in my 4s.